1. Field of the Invention
Certain embodiments of the invention relate to signal processing for navigation satellite systems (NSS). More specifically, certain embodiments of the invention relate to a method and system for calibrating group delay errors in a combined GPS and GLONASS receiver.
2. Background Art
The Global Positioning System (GPS) and the Global Orbiting Navigation Satellite System (GLONASS) are two Global Navigation Satellite Systems (GNSS). GNSS receivers may normally determine their position by receiving satellite broadcast signals from a plurality of satellites.
A fully operational GPS comprises up to 24 earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. Each GPS satellite transmits L-band carrier signals continuously in two frequency bands centered at 1575.42 MHz and 1227.6 MHz, denoted as L1 and L2 respectively. The GPS L1 signal is quadri-phase modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as a coarse/acquisition code (“C/A code”) and a precision ranging code (“P-code”). The GPS L2 signal is BPSK modulated by only the P-code. The C/A code is a gold code that is specific to each satellite, and has a symbol rate of 1.023 MHz. The unique content of each GPS satellite's C/A code is used to identify the source of a received signal. The P-code, is a relatively long, fine-grained code having an associated clock or chip rate of 10 f0=10.23 MHz. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of the P-code used for a given GPS satellite has, a length of precisely one week (7,000 days) before this code portion repeats. The GPS satellite signals comprise navigational information of the transmitting GPS satellite which may be exploited by a corresponding satellite receiver to determine its own navigation information such as the satellite receiver's position and velocity.
The GLONASS system uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. The GLONASS system transmits L-band carrier signals continuously in two frequency bands, denoted as L1 and L2, respectively, centered at frequencies of f1=(1.602+9 k/16) GHz and f2=(1.246+7 k/16) GHz, where k (=1, 2, . . . 24) is the channel or satellite Lumber. Each GLONASS satellite transmits signals in frequencies that are specific to each satellite. The GLONASS L1 signal is modulated by a C/A-code with a chip rate=0.511 MHz and by a P-code with a chip rate=5.11 MHz). The GLONASS L2 signal is BPSK modulated by only the P-code. The P-code is the same, and the C/A-code is the same, for each GLONASS satellite. The GLONASS satellite signals comprise navigation information of the transmitting GLONASS satellite which may be exploited by a corresponding satellite receiver to determine its own navigation information such as the satellite receiver's position and velocity.
Both the GPS system and the GLONASS system use transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.